Bottlehead Forum
Bottlehead Kits => Stereomour II => Topic started by: lskiii on April 23, 2021, 01:03:15 PM
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Hello
So, I'm having an issue that is isolated to just the left channel of my Stereomour II, and I'm wondering if it might be my shunt regulator...
To start - I have a Steremour II w/ all upgrades (Shunt, DC Filament and attenuator) and it normally sounds great. Today however I noticed that the left channel seemed very faint.
Things I tried:
Swapped tubes (no difference)
Directly connected one speaker to the left, then unplugged and plugged it into the right (no difference)
Directly connected my DAC to the amp and tried different RCA cables in Left/Right (no difference)
After none of that worked, I felt I'd narrowed it down to the amp so I did a resistance check, voltage check and looked over everything - it all seemed normal.
Then, I did the tests for the DC Filament (checked out) but the Shunt Regulator tests came back off (and asymetrical).
My readings:
IA 301
OA 227
KREG 1.15
IB 392
OB 338
KREGB 4
So, my hunch is that's the problem, but I'm not sure where to go from here. Any suggestions?
One other clue - I've noticed that when I shut down the amp while the left channel is hooked up, there's a sudden burst of volume right as it's shutting down. It's almost like it momentarily pops up to the proper volume as the power is being cut.
Thank you!
Louis
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Can you give IB and OB on the small PC board as well?
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IB = 391
OB = 339
and, in case it helps:
IA = 301
OA = 209
Thanks!
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So IA on the big board is way out of whack. It could be the SR board, or it could be something else. On the bigger PC board, heat up the red wire that feeds IA, then slip it out and leave it poking straight up, then carefully measure the DC voltage on that wire with the amp running.
If it pops right up to 390-ish volts, we will need to debug half of your SR board.
If it stays at 301V, there is something else wrong with the rest of your amp.
Can you also measure and let me know what the R2 resistor value is on the large PC board?
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So, the red wire, once disconnected, measured 400, so I guess that helps to isolate things.
I'm going to have to re-connect it right now and will test R2...
Thanks!
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OK - I reassembled everything and tested the 2 resistors at R2 on the big board. The one on the "A" side tested 250. However, the one on the B side showed no resistance to ground. Also, the LEDs lit up when I touched the other side of the resistor.
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One other thing - w/ power off, tested resistance across both of the R2s and they both registered 222.
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Yes, when measuring resistance you need to have the amp powered off.
If the LEDs are reacting to you poking around, often that means there's a flaky connection or a broken wire.
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Got it.
So, powered off, the R2 measurement was OK from what I can tell. Anything else that I can do to troubleshoot?
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What I would do next is to put the wire you removed back, then remove the wires going to IA and IB on the smaller PC board. You can just leave them poking up next to the board. Then fire up the amp and see if your OA voltage pops back up.
That would be the final test in my mind to narrow down which piece of the amp is giving you problems.
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OK - So, I've disconnected the 2 red wires that go to IA and IB on the small board.
Now, here's what I'm reading:
Small Board
OB 852
OA 530
IB 700
IA 700
Large Board
OB 300
OA 372
IB. 398
IA. 1.3
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Large Board
OB 300
OA 372
IB. 398
IA. 1.3
Can you check again? IA cannot be lower than OA. How about the Kreg voltages?
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Sorry about that.
I just redid the test - here are the correct #s:
Large Board
OB 300
OA 402
IB. 401
IA. 404
KREG A 1.2
KREG B 2.5
Thanks!
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OK, so what needs to be worked out now is why the shunt regulator isn't working properly. 1.2V of bias and the 400V on OA would mean about 30mA of current being sucked down by the 12AT7, but that isn't happening because your IA voltage would drop like a stone in the process.
In the stock build there's a 220 ohm resistor that goes from the center post of each 9 pin socket to pin 7 or 2 depending on which socket you're looking at. I would be checking those to ensure that this part isn't loose. I would also make extra sure that both halves of the 12AT7 on the problematic side are glowing.
It would also be a decent idea to post some photos of your build.
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Hi,
I've tried a couple of times to upload photos, but I get a website error. Perhaps the files are too large?
Anyway, I have tested the resistor that goes from the center post of each 12AT7 socket. Both register 217 ohm from the center post to pin 7 or 2.
In case it helps, my hunch has been that the issue lies somewhere in the 9 pin socket for the left channel pre-tube. The reason I think that is that it's the only part of the amp that I've messed with. The shunt has been installed for over a year now and I've never had any problems. The left channel, however, has developed a hum and in trying to trouble shoot it I did try to create more space between the various connections that go to that socket. Perhaps that did something?
I will try again to post the photos.
Thanks!
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Yes, you may need to shrink the photos a little.
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Image 1 of 3
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Image 2 of 3
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It would be a good idea to remove enough wires from the SR PC board to be able to photograph the bottom of it as well.
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Close ups of the 2 boards.
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2 of 4
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3 of 4
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4 of 4
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I would go ahead and replace the TL431 on the offending side and see if that helps the situation.
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I may be out of my depth here, but from the manual, I believe you're referring to the LM431A regulator? I'm not sure how exactly I would go about replacing that. i.e. Where would I get the part?
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You can grab one from Mouser or e-mail replacementparts(at)bottlehead(dot)com about getting a new one. The one we use is the LM431ACZ.
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Sounds good - I purchased on mouser and will respond back once done. Thanks. Louis
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Hello - so I finally got around to trying to replace the LM431. Sadly, I believe I trashed the PC board in the process... I removed the old LM431 but in trying to clean out the old solder to install the new, I must have used too much heat because one of the leads to the whole came loose from the board.
Is it possible for me to purchase a replacement big board and parts or do I need to purchase a whole new upgrade kit? Based on my skills, it's much easier for me to populate a PC board than it is to remove/replace old components - esp. one that has 3 prongs that are close together - so just a new board wouldn't work. I would need the new large board + components...
Thanks.
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Yes, you would just need to e-mail replacementparts(at)bottlehead(dot)com.
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Hello
So, today I got the replacement shunt regulator and installed it.
The good news is that the new measurements are all correct - the bad news is that the problem hasn't gone away.
OB 298
OA 298
IB. 392
IA. 392
KREG A 4.0
KREG B 3.7
When I was reassembling the shunt regulator I followed every step and I noticed a problem - I had incorrectly switched the white wire going to C8 and the red wire going to C6. So, correcting that I now I have the white wire from KREG A going to C8. I believe that I messed this up when I was re-soldering all the leads going to "C". I was doing that because I thought that my problem (very weak signal in the left channel) might have been due to an issue there.
So, I suspect that in trying to trouble shoot my issue with the left channel is when I accidentally swapped those 2 wires which led to the messed up shunt regulator readings.
Regardless, I'm kind of back to the original issue. Right channel sounds great. Left channel is weak, has some distortion and gets louder when I power off (and then it fades). I've swapped tubes, source, etc.
Any suggestions?
Thanks,
Louis
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Are the voltages on the 4 pin socket on that side still solid? If so let me know and I can run you through how to temporarily install some jumpers to narrow things down better.
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I will check, but one other thing I wanted to ask - is it possible that the issue could be the tube socket itself? I ask this because:
The left channel has been giving me issues for a while now (I've had the amp for a year). It started with intermittent hum, which over time became intermittent crackling. I swapped out many tubes to try and sort out the source of the issue. Ultimately it became what it is now, which is a weak signal in the left channel that won't go away.
When I was closely looking for any differences between the 2 channels, I noticed that the solder tabs on the left channel socket are much more wobbly than the right channel socket (esp C6 and C8).
I don't know much about how the sockets are internally structured but is it possible that there's a loose internal connection?
Thanks!
Louis
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I believe that these were the readings you were looking for -
A1 57.4
A2 367
A3 14.7
A4 59.9
D1 60.2
D2 367
D3 15.2
D4 57.8
All looks good to me, yes?
Thanks!
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On A3 and D3, is that 15mV, or 15V?
If you run a wire temporarily between A3 and D3, does sound now come out properly from both sets of outputs?
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Yes, A3 and D3 those were 15mv.
So, I ran a wire between A3 and D3 and played the amp.
I ended up playing some mono tracks to test it. One channel seemed to have more bass than the other, but they seemed at a similar volume. The sound was a little bit distorted and there were some crackling/static at points. When I powered it down there was a big surge in volume and then it faded quickly.
Does that help?
Thanks,
Louis
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Did both channels exhibit the exact same behavior?
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Yes, I think. Should I try it again and listen for something specific?
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Can you describe the distortion you heard?
Is the "more bass" on the channel that was otherwise working normally or the one that was not?
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OK - spent some time doing much more detailed testing.
The last post I did, when I experienced distortion, I had moved my amp to a different system. This other system has less efficient speakers (86db vs Klipsch Heresy which is what I normally use with the stereomour. So, I wanted to remove that as a variable.
So today I tested w/ the less efficient speakers first, here's what I heard:
Source in all cases was a mono Beatles recording.
With just one channel (RCA from DAC) The right channel on it's own sounded normal to me. Moving the single RCA input to the left channel, I lost all the bass. Essentially could no longer hear the bass guitar lines.
I then plugged in both RCA channels from the DAC. Volume increased dramatically and the sound was badly distorted in both channels. To me, it sounded like a flubby overdriven distortion that you might get in a guitar amp. I turned the volume down 9db on the stepped attenuator and the distortion went away but the sound still sounded off (muddy bass, not alot of treble)
To see if there was still a difference in the bass between the 2 channels, I disconnected the left speaker, and then the right speaker to A/B them. Both speakers sounded the same to me.
Just for the sake of it, I repeated the above steps with inputs 1, 2 and 3 and in all cases the results were the same.
One other thing - when I turn the amp off, there's a sudden and noticeable volume spike. I don't recall that ever happening until this most recent issue (of a very weak L channel) came up.
Then I moved the amp back to the system w/ the Heresy's and repeated the above test.
In this case, the loss of bass when running just the input into the L channel vs R was still there. However, distortion, even with the attenuator completely wide open, went (almost) completely away.
When running both left and right inputs, the overall audio quality still was off. The bass was over emphasized and "flubby" and the treble was muted. I have an A/B amp switcher (on/off/on) and I tested an NAD amp against the Stereomour and the differences were extreme. The NAD sounded correct, the Stereomour was muted/flubby (which is not at all how it normally sounds to me).
I hope that helps?
Thanks
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With the jumper between pins 3 on each 4 pin socket, repeat the test but run the tube with a 12AT7 in one socket and listen, then shut the amp down and move the 12AT7 to the other socket and retest.
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Here are the results:
Tube in Socket B only - sound from both speakers - weak volume, but sound seems relatively normal
Tube in Socket C only - sound from both speakers - louder volume, but very flubby bass / muddy sounding
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We will have to address the lower volume with just a tube in socket B only. Leave the jumper wire in that you have now between A3 and D3, then add another jumper between terminals 10 and 11. With the two jumpers in, let me know if the volume level coming out of the amp is evened out.
-PB
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I added the jumper between 10 and 11 and retested. Same results. (Tube in B = weak volume / Tube in C = louder but flubby w/ weak treble)
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Well, that would have me suspecting that you have a broken wire or a bad solder joint on or around the 9 pin socket on the offending side.
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So, I re-soldered pin 9 on the C socket (the left channel) and when I tested it, both the L and R channels sound the same (which is good). Thinking that the problem was solved, I removed the jumper between the 3 pins and the 10/11U and the problem came back (L channel is weak). So, I re-installed the jumpers and the problem went away again.
What should I try next?
Also - I hope this helps - I keep suspecting that there's a problem with the C socket. While music was playing just now, I touched the tube in the C socket and got a ton of crackling noises. If I do the same thing to the tube in the B socket, nothing happens. I also noticed that if I applied light pressure to the C socket that there was a large volume drop. Again, doing the same on the B socket produces no change.
To go back to the very beginning of this thread, I had suspected all along that there's something wrong w/ the C socket - it's either a bad solder joint or something with the socket itself. So, before even opening this case, I re-soldered the whole C socket. When doing that, I believe I accidentally swapped the wires going to the shunt regulator. I believe that this is why I started this thread thinking that the issue was with the shunt regulator (as it was measuring wrong).
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Now try just the A3/D3 jumper, then just the 10/11 jumper.
If the amp is OK with the A3/D3 jumper but not ok wit the 10/11 jumper, then you have a driver issue (9 pin socket) on that channel. Typically when your DC voltages are OK but the sound is not,, that's a soldering problem or a broken wire that's just barely making contact where it's broken. Sometimes these issues can be revealed by playing music through the amp and poking around with a wooden chopstick to see if pressing on any particular area dramatically changes the sound or causes pops. This can help focus your efforts quite a bit.
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So here's what I found:
Jumper on 10/11 only - L and R sound the same
Jumper on 3/3 pins - L is weaker than R
Sounds like that narrows down the situation a bit? I've visually inspected the L channel wiring many times and reflowed anything that looks fishy to me. I'm open to any suggestions.
Thanks!
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Not sure is this helps, but I also just tried it with no jumper at all.
No jumpers, Left is substantially weaker than Right. Right sounds great.
3 - 3 jumper, Left is weaker than Right but not as extreme. Also, Right sounds very bass heavy.
I tried reflowing a bunch of connections on the left channel just now. Still no change.
I guess my last hope is to tap on every connection on the left channel while the system is playing?
Thanks
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These results now seem to contradict what you were experiencing before. If just a jumper between 10 and 11 appears to solve your problem, then you have a wiring/solder problem between the RCA jacks and the amp itself. This could be something like a loose resistor in your attenuator if you have that upgrade or a bad solder joint on one of the pots. Jumpering 10 and 11 combines L+R outputs from the input selector and attenuator.
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I figured it made sense to start over a bit, so I did a full resistance and DC test and I think I might have found something.
Every test checked out except for 14. I was reading 260. I looked around at what that's connected to and it's connected to the small board. On the small board there are 2 heat sinks. The heat sink that goes to 02B was completely cold to the touch, while the other was warm.
Would this offer any clue as to where to look? I'll try re-soldering the small board right now just in case.
Thanks
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That feeds terminal 14 then down to the 9 pin socket on that side to pin 1. If you have the 10/11 jumper, does that DC voltage change?
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10/11 jumper - no change, still reading 260 at 14.
Just for the heck of it, did the A3 / D3 jumper and it was still 260.
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For you to get 260 there, here are some possibilities:
1. Bad solder joint or missing jumper between pins 4 and 5 on the 9 pin socket.
2. Loose cathode resistor going to pin 3 on the 9 pin socket.
3. Missing ground reference from attenuator issues (ruled out with the 10/11 jumper).
4. Bad solder joint on the small PC board itself, especially the center joint of the MJE5731A.
5. Broken wire getting from the "O" terminal to the 9 pin tube socket itself.
What DC voltage do you see at pin 3 on that 9 pin socket?
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I measured 2.53V on pin 3. Same reading with or without the jumpers.
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1. Bad solder joint or missing jumper between pins 4 and 5 on the 9 pin socket.
I just did a resistor test and both pins 4 and 5 go to ground, and the solder looks really solid.
2. Loose cathode resistor going to pin 3 on the 9 pin socket.
Is this the resistor from the center of the socket to pin 3? If so, I did a resistance test and it's .366
3. Missing ground reference from attenuator issues (ruled out with the 10/11 jumper).
4. Bad solder joint on the small PC board itself, especially the center joint of the MJE5731A.
I reflowed the solder, but it looks good to me. Is there a way to test the MJE5731A?
5. Broken wire getting from the "O" terminal to the 9 pin tube socket itself.
Did a resistance test between the O terminal on the small board and pin 1 and it's a good connection.
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You can flip the small PC board 180 degrees and reconnect it to see if the problem is on the board or off the board.
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I think we're getting there -
I flipped the board, and now 14 (OA) measures 218 and 7 measures 263.
7 is now going to OB and 14 to OA. I flipped IA and IB as well w/ 9 and 12. They measure the same - 297.
Seems like the issue followed the A side of the small board? Would replacing the small board be the thing to do?
Thanks!
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I would reflow the joints on the transistors. When you get high voltage out that is very frequently a cold joint on the center leg of the MJE5731A. As a last resort you could ask replacement parts about getting a new board, LEDs, transistors, and resistors. Just replacing the transistors on that side would also likely do the job.
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I just tried reflowing and that didn't do the trick.
As a last step, I just checked the sound and the weak channel is now on the right side. So, clearly the issue is somewhere on that small board.
I'm going to just buy the parts and swap it out.
Thank you for your help and patience.